The present invention relates to a method and apparatus for generating halftone colored graphics or displays by applying dots of different colors pixelwise onto a receiving surface, and deals more specifically with the production of polychromatic halftone pixels by applying dots of substantially equal size in random dot patterns to pixel areas of the receiving surface.
The printing or similar generation of halftone colored graphics or displays using variable diameter dots of different primary colors is generally well understood in the printing art. One method commonly used to produce such graphics is that of generating for each of the different primary colors a series of variable diameter dots precisely spaced from one another by a set of screens to print a constant dot placement pattern onto the receiving surface. A printed color reproduction produced by this method generally consists of superimposing the number of different primary colors used of individual images where each of these images varies in density according to the dot size. The saturation (colorfulness) or tone value of a given primary color and density, which is related to the light-reflecting properties of the individual images and is a measure of how dark a color appears, are a function of the percentage area of the receiving surface covered by the dots and are controlled by regulating the size of the dots applied.
The generation of halftone colored graphics using the aforementioned method is generally not practical where dots are applied to dot positions in a matrix format of pixel areas of a receiving surface by ink-jet printing heads nor generally satisfactory in some cases, such as, for example, where the size or diameter of the dot to be applied cannot be varied.
Another problem often encountered with the production of color graphics is the appearance of repeating or moire patterns in the finished halftone color graphic due to the interaction of the dot patterns of the individual screens. Moire patterns also often appear in the finished graphics when dots comprising a halftone of desired hue and saturation level are arranged in the same dot pattern pixel to pixel.
U.S. Pat. No. 4,367,482 to Heinzl, issued Jan. 4, 1983, discloses a method for producing polychromatic halftone images by printing equal sized dots of different primary colors with each color having a fixed, prescribed pattern of dot placement for each saturation level.
The method of producing halftone pixels described by Heinzl is generally unsatisfactory in applications where the recording medium receiving surface may be subject to vibration or other such erratic movement as the medium moves past a dot applying station so that the printing of dots within a pixel and adjacent pixels cannot generally be accurately controlled to produce the desired receiving surface coverage.
It is therefore a general aim of the present invention to provide a method and apparatus for producing polychromatic halftone pixels using ink-jet printing heads to apply color dots of substantially equal size to pixel areas to form color graphics which overcome the limitations and disadvantages of previously used methods and apparatus.
It is a further aim of the present invention to provide a method and apparatus for producing polychromatic halftone pixels which substantially eliminate the appearance of moire patterns in the finished color graphics by applying random dot patterns to adjacent pixel areas so that repetitive dot pattern placement from pixel area to pixel area is unlikely.
Other features and advantages of the present invention will become readily apparent from the following written descriptions and the drawings forming a part thereof.